The ability of platelets to rapidly attach to vWF immobilized at sites of vascular injury, and event paramount to maintaining vascular integrity, is reliant upon the interactions between GPIb alpha and the A1 domain of vWF (vWF-A1). To date, considerable progress has been made in understanding the structural relationship between this receptor-ligand pair, but more effort is required to elucidate the dynamics of platelet adhesion encoded by the physical chemistry of these adhesion molecules and the role that platelet shape plays in facilitating the process of hemostasis. The former is made evident by function-enhancing mutations associated with either molecule, termed platelet type and type 2B vWD, respectively, which modify interactions between this pair of adhesion molecules through distinct mechanisms as determined by analysis of the crystal structure of the complex. Only a detailed kinetic evaluation, however, of the impact of these mutations on bond formation was able to demonstrate that they share common biophysical attributes, a similar enhancement in on-rate and prolongation the lifetime of this interaction. Based on our studies, we hypothesize that the intrinsic on and off-rates of this interaction are key to determining the time and place where platelet-vWF interactions can occur. Our current goal is to build on these concepts by further defining the kinetic properties of the GPIb alpha-vWF-A1 bond as well as determining the impact of platelet shape on adhesion. In Aim 1, we will assess the impact of cell size and shape on the force-driven kinetics of this bond using microspheres, recombinant murine and human vWF-A1 proteins, and platelets from animals deficient in betal-tubulin. Results will be used to develop a computer model designed to replicate platelet-vWF interactions. In Aim 2, we will identify key structural elements within the murine vWF-A1 that promote interactions with GPIb alpha. Site directed mutagenesis of candidate residues will be performed and the kinetics of recombinant proteins ascertained in flow studies and by dynamic force spectroscopy. In Aim 3, we will generate mice with kinetically altered A1 domains, based on results in aim 2, to establish the contribution of the biophysical properties of this interaction in regulating platelet adhesion in vivo. Information from these studies may aid in the design of antithrombotics with specific kinetic properties that can compete with the native-ligand pair.